Amplifying transistor device for regulating circuits



Nov. 26, 1968 H. RUCHARDT ETAL 3,413,530

AMFLIFYING TRANSISTOR DEVICE FOR REGULATING CIRCUITS Filed Nov. 30, 19652 Sheets-Sheet 1 Fig.2

a: n I219) |o 1968 H. RUCHARDT ETAL 3,413,530

AMPLIFYING TRANSISTOR DEVICE FOR REGULATING CIRCUITS Filed Nov. 30, 19652 Sheets-Sheet 2 Fig.6

United States Patent 3,413,530 AMPLIFYING TRANSISTOR DEVICE FORREGULATING CIRCUITS Hugo Riichardt, Gauting, and Hans Hargasser andWinfried Meer, Munich, Germany, assignors to Siemens Aktiengesellschaft,a corporation of Germany Filed Nov. 30, 1965, Ser. No. 510,571 Claimspriority, application Germany, Dec. 1, 1964,

6 Claims. Cl. 317-235 ABSTRACT OF THE DISCLOSURE An amplifyingtransistor device for reducing the effects of cross modulation, having acollector region on one face and having respective emitter and baseregions with respective emitter and base electrodes located on theopposite face. An auxiliary electrode forming a barrierfree junctionwith the base region is located near the emitter at its side away fromthe base electrode. A voltage source connected across the auxiliary andbase electrodes establishes a transverse field in the base region whichprovides a current displacement for the charge carriers injected fromthe emitter region into the base region. The dopant concentration (N inthe collector region sufiices to cause a collector current of at most 10ma. to correspond to a current density larger than the product of v -N-q, wherein v denotes the limit velocity for the charge carriersinjected from the emitter region into the base region at high fieldstrength, N is the dopant density in the collector region, and q denotesthe elementary charge. As a result, an increase in collector currentreduces the amplifying gain.

Our invention relates to amplifying transistor devices for use inregulating circuits and has for its principal object to minimize thenon-linearity of the transistor amplifying characteristic to therebyreduce cross modulation.

The invention will be described with reference to the accompanyingdrawing in which FIGS. 1-3 are explanatory graphs;

FIG. 4a shows by way of example a regulating transistor according to theinvention by a plan view, and

FIG. 4b is a corresponding side elevation;

FIG. 5 is a plan view of another embodiment of such a transistor;

FIG. 6 is a graph of measuring results;

FIG. 7 is the diagram of a measuring circuit employed for obtaining themeasuring results represented in FIG. 6; and

FIG. 8 is a plan view of another embodiment of a transistor according tothe invention.

A high-frequency transistor amplifier may exhibit cross modulation dueto :a non-linear input characteristics. Cross modulation is thepheonmenon that when a signal from a transmitter (useful or maintransmitter) is being received the modulation of a different transmitter(interfering or transient transmitter) operating on a different carrierfrequency appears superimposed as noise upon the useful signal, whereasthe modulation of the transient transmitter is not discernible when thecarrier wave of the useful signal is absent. Such interference,therefore, cannot be eliminated by any selective circuit means, such asintermediate-frequency filters, in the receiving circuitry.

This poses the problem of linearizing the input characteristic of anamplifying transistor within a limited control range. The problem isaggravated by the requirement that the transistor should furnish aregulatable power amplification.

The power amplification of a regulating transistor depends upon theoperating point adjusted within a given operating characteristic.Regulatability of a transistor is constrainedly accompanied bynon-linearity of the input characteristic. Regulating transistors areoperated in regulating circuits, in which a change in amplification isobtained by the fact that the transconductance decreases with increasingemitter or collector current.

High-frequency regulating transistors are supposed to attain a highestfeasible level of permissible noise voltage within the desired workingrange. The term level of permissible noise voltage denotes the amount ofsignal voltage required of the transient transmitter to effect a 1%modulation of wave. i

The non-linearity of the amplification of a transistor is expressed bythe function V =F(I A substantially similar relation also applies to thedifferential transconductance. The relation between output current andinput voltage is satisfactorily approximated by a series of exponentialterms:

DU 1! 2 m3 wherein U denotes the input signal voltage. When measuringcross modulation, this signal U, supplied to the transistor, isfurnished from a non-modulated useful transmitter and aamplitude-modulated interfering transmitter. Calculation (not herereproduced) shows that the signal U first reaches at the term of thethird order an output current whose frequency is that of the usefultransmitter and whose modulation is that of the interfering transmitter.Graphically shown in FIGS. 1, 2 and 3 are the transconductance as afunction of collector current for a regulating transistor, the firstderivative and the second derivative of the transconductance function.The absicca in each graph represents collector current I The ordinaterepresents the derivative values shown in each graph at the left. Thetransconductance C/ d U is approximately proportional to theamplification V In FIG 1, therefore, curve 1 also represents in goodapproximation the power amplification characteristic. Curve 2 shows theconfiguration of the permissible spurious voltage U,, likewise as afunction of the collector current 1 U is the voltage of a modulatedinterfering transmitter which produces a defined modulation of a usefultransmitter. The spurious voltage permissible or required for producinga given disturbance (noise) of the signal received from the usefultransmitter is to be as high as feasible within the entire regulatingrange of a regulated transistor. Of particular importance for receiversis the increase in permissible spurious voltage with increasingregulation, in comparison with the operating point at maximalamplification.

The curve 1' in FIG. 2exhibits the first derivative of thetransconductance function which is proportional to the first derivativeof the amplifying characteristic. The curve 1" in FIG. 3 represents thecurvature of the transconductance function which is proportional to thecurvature of the amplifying characteristic. The curvature of theamplifying characteristic constitutes a measure of the cross modulationand hence of the permissible spurious voltage.

It is thus a more specific object of the present invention to minimizeas much as possible the curvature in the amplifying characteristic of aregulating transistor in order to thereby increase the permissible orrequired spurious (noise) voltage over the entire regulating range ofthe transistor.

To this end, and in accordance with our invention, we employ atransistor having an emitter region, a base region and a collectorregion, in which an emitter electrode and a base electrode are arrangedon a surface facing away from the collector region of the semiconductorbody forming the transistor, this body preferably consisting of a waferor platelet. It is essential to the invention that in such a transistorthe dopant density N in the collector region is so chosen that acollector current I of a few milliamps, namely at most 10 ma., causesthe current density j to be larger than the product of v N q, wherein vis the limit velocity for the charge carriers injected from the emitterregion into the base region at high field strength (3 to 6-10 cm./sec.),N is the dopant density in the collector region, and q is the elementarycharge. Under these conditions, a change in collector current I altersthe effective width of the base region and hence also the high-frequencyamplifying gain. Furthermore, we provide the transistor with anauxiliary electrode which forms a barrier-free contact with the baseregion and is likewise located near the emitter electrode but at theemitter side away from the base electrode. We provide the base electrodeand the auxiliary electrode with respective terminal connections forelectrically connecting them with each other through a direct voltagesource so that, during transistor operation, a transverse current flowsthrough the base region and effects a current displacement for thecharge carriers injected from the emitter region into the base region.

For a given value of collector current 1 the current density j dependsupon the injecting emitter area. Since the emitter area affects theelectrical parameters of the transistor, particularly the limitfrequency, it follows that the dopant density N will assume differentvalues depending upon the electrical data, particularly the particularfrequency range, for which the transistor is dimensioned.

The transistor may be so designed that the collector region has twoportions of which One is adjacent to the i.

In this equation, r denotes the base spreading resistance; C denotes theportion of the collector capacitance in series with r f is the frequencyof operation, and f y the frequency value when the amplification incommon configuration is equal to unity. The high-frequency amplificationV for a given frequency 1 may be reduced by reducing the numerator or byincreasing the denominator in the equation. For high frequencies, theinput and output impedances of a transistor are largely determined bythe base spreading resistance and the above-indicated share of thecollector capacitance. A change of these magnitudes for the purpose ofregulating the amplification leads to matching changes and to detuningof preceding or following filters. An optimal change in amplification ata lowest change in impedance can be attained by changing the transfermagnitude f At relatively large currents, I is inversely proportional tothe base transmit time:

wherein 1' denotes the base transit time. The carrier transit time is asquare function of the width of the fieldfree base traversed bydiffusion:

Z TLNF Herein D denotes the diffusion constant of the minority chargecarriers in the base space and w the electrically active width(thickness) of the base region. The collector space charge zone, formingin the operation of the transistor and commencing at the collector pnjunction, as a space-charge density pc proportional to the dopantdensity N in the high resistivity region between base and collector. pcis determined by the equation pc NC wherein q denotes the elementarycharge.

Assume that there is injected from the emitter into the base a currentwhich passes into the collector, the current density (i of the injectionbeing jc g NC q wherein v denotes the limit speed for the chargecarriers in the semiconductor at high field strengths amounting to about3-6 l0 cm./sec. Under such opearting conditions, the original collectorspace-charge zone is reversely charged or displaced. This results inwidening the thickness w of the field-free, electrically effective baseregion. It follows from the approximation 4 that the increase ineffective base thickness w prolongs the charge-carrier transit time.This, according to Equations 3 and 2, results in reducing thehigh-frequency amplification. This widening of the base thickness andthe accompanying reduction in high-frequency amplifying gain increaseswith increasing collector current 1 The amount of base widening becomeslarger, the thicker the collector region is chosen. For low powerlosses, as Well as for stability of noise and constancy of impedance, itis further essential that the dopant concentration N in the collectorregion be chosen so low that the base widening occurs with a fewmilliamps of the entire collector current, which is determined by therelation 10 J O jaunt In this equation, F denotes the injecting emitterarea. For uniform injection over the entire emitter area, the Equation 7becomes simplified to I =j F Investigation has shown that by collectorwidening a further reduction in curvature of the amplifyingcharacteristic and thus an increase in permissible spurious voltage, canbe attained by designing the transistor to provide a means for effectinga substantial current displacement for the charge carriers injected fromthe emitter region into the base region. This is achieved by providingthe transistor with the above-mentioned auxiliary electrode which formsa barrier-free contact with the base and is likewise located near theemitter electrode but on the side away from the base electrode, the baseelectrode and the auxiliary electrode being provided with terminals orleads by means of which they are connected with each other through acurrent source when the transistor is in operation. During suchoperation, therefore, a transverse current flows through the base. Thistransverse current may produce a current displacement zone for thecharge carriers injected from the emitter into the base region, sincethe flow potential at the emitter p-n junction, which causes theinjection, becomes a non-constant monotonous function of the locality byvirtue of the transverse current.

The poling of the direct current thus flowing through the base layer orregion is determined by the resulting course of the permissible spurious(noise) voltage for 1% cross modulation as a function of the particularamplifying gain. In principle, positive and negative polarities arepossible. By virtue of the base transverse current supplied through theadditional electrode, the cross-modulation behavior of the transistor isconsiderably improved. A transistor made according to the inventionaffords within a large regulating range an increase in permissiblespurious voltage U and consequently secures a corresponding reduction incross modulation that is largely independent of specimen stray, i.e.inevitable differences between different specimens of a manufacturingseries. As mentioned, the transistors are operated in regulatingcircuits. These can be in common base configuration as well as in commonemitter configuration. The fundamental manufacturing method employed formaking the transistor is likewise of minor significance. For example, atransistor may be produced with or without use of the epitaxial processby mesa, planar or P013 (push-out base) techniques.

The manufacture can be started with a uniformly doped semiconductorcrystalline body possessing the dopant concentration N desired in thecollector region. The known diffusion and alloying methods may beemployed for providing such a crystalline body with the emitter regionand the base region. However, the collector region may also be'pi'oduced epitaxially and in this case generally consists of a lowresistivity portion directly adjacent to the collector electrode and arelatively high resistivity portion which is adjacent to the base regionand possesses the dopant concentration N.

The embodiment of a regulating transistor according to the inventionexemplified in FIGS. 4a and 4b is produced from germanium by the knownmesa technique and has a p-n-p sequence of acceptor and donor dopedregions. The semiconductor body 11 of the transistor has a mesa 12 ontop of which the emitter contact 9 and the base contact 10, as well asan auxiliary electrode 8, each of them strip-shaped, are deposited. Thepresent example has the following dimensions. The length of each strip8, 9, is 11:40 The width of each strip is e=l5 t. The dimensions of themesa 12 are (1:60;; and f=90 The distance between emitter 9 and baseelectrode 10 is c=6 The disstance d between emitter 9 and auxiliaryelectrode 8 is The midpoints of the three electrodes '8, 9, 10 arelocated on a straight geometric line. The midpoint of the auxiliaryelectrodeyhowever, may also be located outside of the connecting linebetween the midpoints of emitter and base electrodes, this being thecase in the embodiment shown in FIG. 8, where the auxiliary electrode isdenoted by '8 and the items denoted by 9, 10, 11, 12 correspond to thosedenoted in FIG. 4a by the same numerals respectively.

Essential to the auxiliary electrode 8 or 8' is the fact that it islocated on the emitter side away from the base electrode 10 on the sameface of the semiconductor crystal. In other respects the auxiliaryelectrode may be placed in any desired manner relative to the emitterelectrode 9. The position of the auxiliary electrode relative to theemitter and the base electrode is so chosen as to secure optimalregulating performance. This, in a particular case, depends mainly uponthe doping of the high resistivity region located between base andcollector, and also upon the magnitude of the base transverse current,this current being in the order of magnitude of milliamps (ma.). Forexample, the transverse current may vary between 2 and 50 According toanother embodiment of the invention, several base electrodes, all orsome of them short-circuited among themselves, may be provided.Furthermore, the additional or auxiliary electrode may simultaneouslyconstitute a base electrode of the transistor. This will be furtherelucidated with reference to the embodiment exemplified in FIG. 5.

According to FIG. 5, the semiconductor crystalline body 20 is providedwith a mesa 21 which carries on its top a number of strip-shapedelectrodes 22, 23, 24, 25, 26. According to an embodiment of theinvention, the two electrodes 23 and 25 may constitute the emitterelectrodes of the transistor and may be short-circuited with each otherby being both connected to an emitter terminal ET as shown. Theelectrode 24 may be considered as the auxiliary electrode, and theelectrodes 22 and 26 are the base electrodes and are short-ciro'uitedwith each other. The base voltage is effective between electrodes 22/26on the one hand, and the centrally located electrode 24 on the otherhand. For this purpose, the base electrodes 22 and 26 are provided withconnector leads which extend to the outside in insulated relation to theother components of the transistor; and the direct voltage required forpassing a base transverse current through the transistor is connected 6between the electrodes 22/26 and 24 as shown. The electrodes 22 and 26become less effective to function as a base electrode as the transversecurrent increases.

The diagram of FIG. 6 exhibits results obtained with transistors asdescribed above with reference to FIGS. 4

and 5. The abscissa indicates collector current 1 in ma.,

and the ordinate denotes voltage. The full-line curve 5 represents theamplifiaction characteristic V The fullline curve 6 indicates thepermissible spurious voltage U These curves were taken with a regulatingtransistor according to the invention in which the transverse currentpassing through the base region was 10 ma. In comparison thereto, thebroken-line curve 3 indicates the amplifying characteristic, and thebroken-line 4 the permissible spurious voltage for a base transversecurrent zero.

The measuring results shown in FIG. 6 were obtained with the aid of ameasuring circuit, as illustrated in FIG. 7. Denoted by T in FIG. 7 isthe transistor in base configuration. The signal arriving at the input Estemmed from a useful transmitter having a frequency of 200 mHz.(megacycles) which was not modulated, so that only the carrier frequencywas received, and also from a transient transmitter having a frequencyof 210 mHz. amplitude modulated with one kHz. ('kilocycle). The totalinput resistance R was 60 ohms. The useful transmitter signal at theoutput A of the measuring circuit was 1% amplitude modulated by spurious(noise) voltage. The load resistance was 60 ohms and was transformed bymeans of a tank circuit S up to 900 ohms, for thus matching it to theoutput of the transistor T.

The resistors R and R of which R is adjustable, form a voltage dividerfor the collector voltage supplied from a battery B whose voltage is 12v. Denoted by C and C are coupling capacitors. Further capacitors C andC serve blocking purposes. A battery =B supplies the base transversecurrent to the base electrode and the auxiliary electrode through anadjustable potentiometer rheostat R The resistance of R R R is 1K ohmeach.

We claim:

1. An amplifying transistor device for regulating circuits, comprising asemiconductor body having a collector region on one face and havingrespective emitter and base regions with respective emitter and baseelectrodes located on the opposite face, an auxiliary electrode formingwith said semiconductor body a barrier-free junction on said oppositeface and being located on said base region near said emitter electrodeat its side away from said base electrode, direct-voltage supply meansconnected to said base electrode and said auxiliary electrode forpassing a transverse current through said base region, means forreducing said transistor high frequency amplifying gain with increasingcollector current, said means comprising a dopant concentration N in thecollector region such that at a collector current of at most 10 ma. thecollector current density j is greater than v -qN wherein v denotes thelimit speed for charge carriers in the semiconductor at high fieldstrengths, said limit speed amounting to about 3-6-10 cm./sec.., Ndenotes the dopant density in the collector region, q denotes theelementary charge.

2. In a transistor device according to claim 1, said auxiliary electrodehaving its midpoint spaced from the common geometrical centerline ofsaid. emitter electrode and base electrode.

3. In a transistor device according to claim 1, said auxiliary electrodehaving its midpoint located on the common geometrical center line ofsaid emitter electrode and base electrode.

4. A transistor device according to claim 1, comprising a plurality ofcomponent base electrodes spaced from each other and electricallyshort-circuited with each other.

5. In a transistor device according to claim 1, said direct-voltagesupply means to provide a base transverse current of about 2 to about 50ma.

6. An amplifying transistor device for regulating circuits, comprising asemiconductor body having a collector region on one face and havingrespective emitter and base regions with respective emitter and baseelectrodes located on the opposite face, an auxiliary electrode formingwith said semiconductor body a barrier-free junction on said oppositeface and being located on said base region near said emitter electrodeat its side away from said base electrode, direct-voltage supply meansconnected to said base electrode and said auxiliary electrode forpassing a transverse current through said base region, said transistorhaving a dopant concentration in the collector region such that at acollector current of at most 10 ma. said transistor exhibits a declininggain characteristic with increasing collector current.

References Cited UNITED STATES PATENTS 2,924,760 2/ 1960 Herlet 3 l72353,087,098 4/ 1963 Taylor 317235 3,226,613 12/1965 Haenichen 317235 OTHERREFERENCES Characteristics and Circuit Applications for a Tetrode PowerTransistor by Marshall, Feb. 17, 1956, pp. 1-7 relied on.

JOHN W. HUCKERT, Primary Examiner.

J. D CRAIG, Assistant Examiner.

